Introduction b 6 f 2004 f f f 1992 c- 1994 f 1983 2004 1978 1985 1994 Nostoc 2000 2003 2003 f 2004 2005a 2005 2005 f f f 2004 2005a 2005 2005 f Materials and methods Protein samples Nostoc E. coli 1998 Nostoc f 2005 f f −1 −1  1998 −1 −1 f 2005 f f  III f f  II f  III II f II f  III II f  III f 2005a I f  II II f  III XAS measurements The X-ray absorption spectra were recorded at the European Synchrotron Radiation Facility (ESRF) in Grenoble (France), which was operating with a ring current of 200 mA and energy of 6 GeV. The Fe K-edge (7112 eV) was measured at beam line BM29 using a double crystal monochromator fitted with a pair of flat Si (311) crystals detuned to 50% fwhm of the maximum transmission, for the suppression of high energy harmonics. All measurements were made at room temperature in fluorescence mode, using a 13-element Canberra solid-state germanium detector. The samples were contained in a PTFE cell equipped with two 12 μm Kapton foil windows. Energy calibration was achieved by measuring an iron foil for the Fe edge. The spectra of the foil were measured simultaneously, with the data in transmission mode, to be used as an internal reference. Ionization chambers filled with the appropriate mixture of gases were used as detectors. f XAS data analysis E 0 1992 1995 1995 Results and discussion XANES region 1 upper panel f s− d 1997 f 1994 Fig. 1 f Upper f  III f  II Middle f  III II f  III Lower f  II I f  II  f  II f  III c 1999 Nostoc c 6 2006 c 1999 2+ 3+ c 1976 Moreover, there are some differences in both XANES spectra, particularly in the first feature after the edge. 1 f II f  III f  III 1 middle panel 1 lower panel I f  II f II f II I f 3 f EXAFS Region f 2 left panels k k 3 k k 3 k 3 Fig. 2 f f  II f III panel a f  II f  III panel b I f  II II f  III f f  II f III panel c Panel d f I f  II II f  III f  II I f  II f  III II f  III  2005a b f f 1 2 right panels Phormidium f 1999 ε2 r 1 r 2 2 1 2  2 2 E 0 Table 1 f System Ligand E 0 r(M–L) (Å) 2 2 f III N (NC) −7 ± 4 1.96 ± 0.02 0.002 ± 0.001 N (NA & ND) 1.97 ± 0.02 N (NB) 1.98 ± 0.02 ε2 2.44 ± 0.06 0.010 ± 0.010 N (N) 2.49 ± 0.06 f II N (NC) −6 ± 2 1.97 ± 0.01 0.002 ± 0.001 N (NA & ND) 1.99 ± 0.01 N (NB) 2.00 ± 0.01 ε2 2.43 ± 0.02 0.004 ± 0.003 N (N) 2.48 ± 0.02 II f III N (NC) −7 ± 3 1.97 ± 0.01 0.002 ± 0.001 N (NA & ND) 1.98 ± 0.01 N (NB) 1.99 ± 0.01 ε2 2.40 ± 0.03 0.011 ± 0.009 N (N) 2.45 ± 0.03 I f II N (NC) −6 ± 3 1.97 ± 0.02 0.003 ± 0.001 N (NA & ND) 1.98 ± 0.02 N (NB) 1.99 ± 0.02 ε2 2.40 ± 0.03 0.010 ± 0.009 N (N) 2.45 ± 0.03 0 2 0 2 k R f  III R f  II II f  III R I f  II 2+ f  II I 2+ f  III II 3 f 2+ f  II I 3+ 2+ 3+ 2+ 1999 2+ I 2+ 3+ II f  III Fig. 3 f III I f II f  III I f  II  f II I d z d x y c 6 2006 d c 6 f. f